1. Field of the Invention
The present invention relates to an apparatus and a method for producing a depressed-cladding core rod of an ultra-low water peak optical fiber. The method avoids contamination of the interface and the inside of the core rod by moisture and impurities in the air.
2. Description of the Related Art
Process of producing optical fiber normally includes three steps. The first step is producing the core rod. The core rod usually consists of two parts: the core layer and the inner cladding layer. The methods of producing a core rod mainly include vapor axial deposition method (VAD), modified chemical vapor deposition method (MCVD), plasma chemical vapor deposition method (PCVD), and outside vapor deposition (OVD). The second step is adding an outer cladding layer outside the core rod. The third step is drawing to produce the optical fiber.
The methods for adding an outer cladding layer outside the core rod mainly include rod-in-tube method (RIT), rod-in-cylinder method (RIC), and outer cladding deposition method. The rod-in-tube method is inserting the core rod into a casing tube, fusing the casing tube and the core rod together in a high temperature to form a solid preform. The rod-in-cylinder method is also inserting the core rod into a casing tube, while the fusing process of the casing tube accompanies with a process of drawing. The outer cladding deposition method takes advantage of technologies such as soot cladding, advanced plasma vapor deposition method (APVD), and plasma outside vapor deposition method (POD) to add an outer cladding outside the core rod to produce a solid preform. In order to control the production cost of the optical fiber, the fraction of the impurities (especially the moisture) in the material of the outer cladding layer is usually higher than that in the core rod, and the physical and chemical properties of the outer cladding are not so good as those of the core rod. Furthermore, deposition of the outer cladding layer is usually performed in a high temperature that affects the quality of the core rod. In order to eliminate the influence of the material and the deposition process on the quality of the core rod, the thickness of the inner cladding layer is usually not smaller than 10 times the wavelength of the propagating light. To satisfy this demand, the ratio of the outer diameter of the inner cladding layer to the outer diameter of outer cladding layer is usually not smaller than 4.
Among the methods of producing a core rod, VAD method is widely used because it has fewer requirements on the purity of raw materials, a relative high deposition speed, and better dehydration effects. VAD method is capable of continuously producing large-sized preform, which has a refractive index profile that does not have depression in the center. However, during the processes of dehydroxylation and sintering, the fluorine doped in the inner cladding layer easily permeates the core layer or evaporates in a high temperature to be taken away by the dry gas (Cl2 and He). Thus, in the core rod produced by traditional VAD method, the difference of the depressed refractive indexes usually does not reach −0.003 (approximate −0.2% of n(SiO2), in which n(SiO2) represents the refractive index of SiO2). Therefore, it is difficult to achieve a complex refractive index profile (e.g., that of an inner cladding layer in shape of concave and convex in turn) using traditional VAD method. This disadvantage limits the application of traditional VAD method to only production of single model optical fiber having a simple refractive index profile without deep depression, such as G652, G657 A1/G657 A2, etc.